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La energía solar es la energía asociada con la radiación solar y representa la fuente primaria de energía en la Tierra.
Desde el punto de vista energético, es una energía alternativa, renovable y limpia respecto a los combustibles fósiles tradicionales y una de las energías que apoyan la llamada economia verde en la sociedad moderna
Puede ser explotada a través de diferentes tecnologías y para diferentes propósitos.

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Se clasifican en pasivas o activas según la forma en que capturen, conviertan y distribuyan la energía solar. Las activas usan paneles fotovoltaicos para recolectar dicha energía, mientras que las pasivas usan técnicas de arquitectura bioclimatica.

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Los combustibles solares convierten la energía del sol en combustible líquido imitando el proceso de fotosíntesis de las plantas: capturar la energía del sol y almacenarla en una serie de enlaces químicos a través de la división del agua y el dióxido de carbono.
La mayoría de los combustibles para el transporte  y  otras materias primas para la industria se producen a partir de carbón, petróleo o gas natural. Pero una ruta alternativa para la producción de combustibles líquidos y gaseosos podría ser el uso de tecnologías que aprovechan la luz solar.

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¿Cómo se obtienen los combustibles solares?

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¿Cómo se obtienen los combustibles solares?

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Una molécula de agua se compone de dos átomos de hidrógeno y uno de oxígeno. Separar los átomos del hidrógeno del oxígeno es un proceso muy costoso, tanto a nivel energético como económico. Por lo tanto, se hace necesario el uso de catalizadores; unos compuestos que rebajan la energía necesaria hasta un determinado nivel en el que la luz solar sea eficiente. Cuando se separan los hidrógenos, se combinan con dióxido de carbono, para obtener hidrocarburos

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Un grupo de expertos logró desarrollar un fluido que puede almacenar la energía solar durante más de una década. El hecho es importante para reemplazar otros combustibles.
Le llamaron “combustible térmico solar”, y es un líquido.
Su forma de actuar, a grandes rasgos, podría asimilarse al de una batería recargable, “pero en lugar de electricidad, se pone la luz solar”.

Internamente, tiene moléculas que involucran carbono, hidrógeno y nitrógeno, aunque cuando son golpeadas por la luz solar, hacen algo inusual: “los enlaces entre sus átomos se reorganizan y se convierten en una nueva versión energizada de sí misma”.
Esto quiere decir que la energía del sol se captura en los fuertes enlaces químicos y permanece ahí.

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Como bien se menciono, los combustibles solares son energías de carácter renovable. En la industria también tiene su papel el cual se mencionara en el sector agrícola:

Una PYME agrícola paso de pagar 160.000 a 20.000 colones en electricidad con paneles solares.
se colocaron 36 en finca de pozos de Santa Ana.
El autor de dicha se llama Abelardo Guillen Marin, cada mes consumía 160.000 colones en electricidad por su sistema de riego y casas junto con su familia invierto 12 millones por 36 paneles fotovoltaicos y dicha inversión ayudo a que ese monto disminuyera drasticamente.
La familia quiso disminuir su huella ecológica.

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Significance

Combining high-throughput computation and experiment accelerates the discovery of photoelectrocatalysts for water oxidation and explains the origin of their functionality, establishing ternary metal vanadates as a prolific class of photoanode materials for generation of chemical fuels from sunlight.

Abstract

The limited number of known low-band-gap photoelectrocatalytic materials poses a significant challenge for the generation of chemical fuels from sunlight. Using high-throughput ab initio theory with experiments in an integrated workflow, we find eight ternary vanadate oxide photoanodes in the target band-gap range (1.2-2.8 eV). Detailed analysis of these vanadate compounds reveals the key role of VO ₄ structural motifs and electronic band-edge character in efficient photoanodes, initiating a genome for such materials and paving the way for a broadly applicable high-throughput-discovery and materials-by-design feedback loop. Considerably expanding the number of known photoelectrocatalysts for water oxidation, our study establishes ternary metal vanadates as a prolific class of photoanode materials for generation of chemical fuels from sunlight and demonstrates our high-throughput theory-experiment pipeline as a prolific approach to materials discovery.

The use of predictive simulation in combination with experiments for the accelerated discovery and rational design of functional materials is a challenge of significant contemporary interest. High-throughput computing and materials databases ( 1- 3), largely based on density-functional theory (DFT), have recently enabled rapid screening of solid-state compounds with simulation for multiple properties and functionalities ( 4- 10). Since their advent just a few years ago, these DFT-based databases and analytics tools have already been used to identify more than 20 new functional materials that were later confirmed by experiments across a number of applications ( 8), motivating concerted efforts to validate theory predictions with experiments ( 11). However, in photoelectrochemistry for the renewable synthesis of solar fuels, efficient metal-oxide photoanode materials--photoelectrocatalysts for the oxygen evolution reaction (OER)--remain critically missing ( 12). Forty years of experimental research has yielded just 16 metal-oxide photoanode compounds with band-gap energy in the desirable 1.2-2.8-eV range that strongly overlaps with the solar spectrum. Prior high-throughput computational screening studies have yet to expand this list ( 6, 7, 13), in part due to quantitative limitations in predictability of the electronic structure--especially band-gap energy, E g, and the valence band maximum (VBM) energy, E VBM--from the chemical composition and crystal structure. Our integration of ab initio theory with high-throughput experiments has yielded a most prolific materials discovery effort, as demonstrated by the identification of 12 water oxidation photoelectrocatalysts in the target band-gap range, including our recently reported 4 copper vanadates ( 14) and 8 additional metal vanadates reported here.

Monoclinic BiVO ₄ ( 15) has received substantial attention as a solar fuels photoanode material due to its promising OER photoactivity. It has a desirable 2.4-eV band gap derived from a conduction band minimum (CBM) consisting of V 3d states, and a VBM of mixed O 2p and Bi 6s character ( 16). We recently identified another ternary vanadate, β-Mn ₂V ₂O ₇, that, although not photoactive for the OER, exhibits a 1.8-eV band gap and valence band alignment to the OER equilibrium energy, or OER potential, a result of hybridization of Mn 3d with O 2p states. β-Mn ₂V ₂O ₇ shares a common VO ₄ structural motif with BiVO ₄: Both compounds possess 3d ⁰ V cations tetrahedrally coordinated by oxygen. Orbital hybridization resulting from this VO ₄ motif engenders significant baseline O 2p and V 3d character at the VBM and CBM, creating an electronic structure "scaffold" that enables the formation of a desirable E g and E VBM upon introduction of an additional metal cation ( 17). For example, in BiVO ₄ and β-Mn ₂V ₂O ₇, additional hybridization of O 2p with Mn 3d and Bi 6s states, respectively, leads to an increase in the E VBM toward the OER potential. Here, we hypothesize that this VO ₄-scaffold phenomenon applies broadly to ternary vanadates. Indeed, we observe that three additional previously known OER photoanodes with band-gap energies between 1.2 and 2.8 eV [α-Ag ₃VO ₄ ( 18), FeVO ₄ ( 19, 20), and β-Cu ₃V ₂O ₈ ( 21)] are also ternary vanadates with a VO ₄ structural motif in the 3d ⁰ electronic configuration. For these reasons, VO ₄-based ternary compounds are fertile ground for both discovering metal-oxide photoanodes and seeding the photoanode materials genome.

Current computational and experimental approaches differ in the material properties they can most efficiently and effectively characterize, and by leveraging their complementarity we can both accelerate the materials discovery process and confirm the validity of each approach. After several generations of integrated computational and experimental materials screening workflows, we arrived at the tiered screening pipeline of Fig. 1 in which we ( i) selectively mine a materials database with a well-defined hypothesis to arrive at a subset of promising materials; ( ii) screen this materials subset for select properties with high-throughput computation at appropriate levels of theory; and ( iii) use companion combinatorial experiments on the same subset to both validate the calculations and characterize material performance under device-relevant conditions. In the present work we demonstrate the efficacy of this approach for the discovery of ternary vanadate photoanodes and note that the strategies outlined herein are broadly applicable for discovery of functional materials.

To evaluate the VO ₄-scaffold hypothesis, the 7 tiers of screening commence with a query of the Materials Project (MP) database ( 1) to identify 174 known VO ₄-based ternary vanadates. Out of these 174 ternary vanadate compounds, 147 compounds are indexed in the Inorganic Crystal Structure Database as previously synthesized materials, and the remaining 27 compounds are the results of previous structure prediction calculations within the MP. For each compound, the database also provides the DFT formation energy above the convex hull in the composition phase diagram (ΔH) and a coarse estimate of E g. These quantities are computed using DFT with the generalized gradient approximation of Perdew, Burke, and Ernzerhof (PBE) and Hubbard U ( 22) corrections for metal cation d states with the Vienna Ab initio Simulation Package (VASP) code ( 23). Our DFT-PBE+U calculations provide a high-throughput tier 2 screen that thresholds ΔH and E g in an effort to avoid nonsynthesizable materials ( 6) and known wide-gap insulators, respectively. To more accurately screen and identify semiconductors with E g in the target range, we note that a higher level of theory would be in principle required, such as ab initio many-body perturbation theory ( 24), at significantly greater computational expense. As a compromise that balances computational efficiency with adequate accuracy, we proceed in practice with DFT but use generalized Kohn-Sham states obtained from the hybrid functional of Heyd, Scuseria, and Ernzerhof (HSE) with a modified mixing parameter, α, of 0.17 (SI Appendix) that results in more tolerant acceptance criteria for tier 3. The HSE functional features improved treatment of short-range exchange and correlation effects relative to PBE+U that treat delocalized sp valent and localized d states on equal footing ( 25) and can lead to more predictive band gaps ( 26). Finally, in tier 4, we evaluate E VBM using a combination of bulk HSE (α = 0.17) and surface slab PBE+U calculations ( 27, 28) to determine if the VBM meets the OER thermodynamic requirement (SI Appendix). Surface dipoles that arise upon interfacing the metal oxides with water are expected to slightly raise E VBM ( 28); evaluating E VBM relative to vacuum, as we do here, is therefore a particularly lenient screening criterion for allowing the ensuing experiments to demonstrate operational VBM alignment via photocurrent measurements.

To validate these screening criteria and their propensity for identifying photoanode materials, we turn to combinatorial experiments. The most common bottleneck in computation-guided experimental work is synthesis of the target materials in a device-relevant format, prompting our development of combinatorial sputtering and annealing methods in which thin-film synthesis is attempted for each target phase (and its off-stoichiometry variants) using a variety of reactive sputtering and annealing conditions. The materials exhibiting target-phase purity in excess of 80% pass to tiers 6 and 7, where high-throughput optical spectroscopy (UV-vis) and photoelectrochemistry characterize E g (direct and indirect transitions) and the photocurrent density at the Nernstian potential (J O2/H2O), respectively. It is important to note that for these final tiers of screening, the optical and photoelectrochemical properties are measured not only for the target phase but also neighboring phases in the composition space, enabling identification of E g and J O2/H2O values that are representative of the target phase. The high-throughput J O2/H2O measurements use sufficiently energetic photons (∼3.2 eV) to excite semiconductors across the entire E g range.

The list of tier 4 compounds is provided in SI Appendix and includes several examples of multiple polytypes with a common formula unit, and often a subset of these polytypes are synthesized in high purity using the combinatorial synthesis techniques. Other phases may not be accessible with these synthesis techniques due to kinetic limitations or the presence of stable phases that are absent from the MP database. In total, 17 high-purity compounds were obtained, comprising tier 5 of the screening pipeline. Band-gap estimation through automated Tauc analysis experimentally verified the band-gap energy range for 16 of these 17 phases, demonstrating the fidelity of our DFT-HSE (α = 0.17) band-gap calculations. The most striking acceptance rate in the screening pipeline is the final screening criterion, where 15 of 16 phases exhibited photoactivity when biased at the OER potential, an exceptionally high hit rate of 94%.

The 15 metal-oxide photoanodes identified by the pipeline are illustrated in Fig. 2, demonstrating that desirable E g and photoactivity are obtained for ternary vanadates with a range of cation elements and respective electronic configurations, a marked departure from the d ⁰ configuration of photoanodes such as TiO ₂ and WO ₃. The band-gap and J O2/H2O values are also listed in Table 1, which includes three photoanodes reported by other investigators, validating the pipeline's ability to replicate known results. Several other phases in Table 1 have been explored as photoactive materials for different reactions or conditions [γ-Cu ₃V ₂O ₈ ( 29), Cr ₂V ₄O ₁₃ ( 30), and β-Ag ₃VO ₄ ( 14) for photodegradation of organics and Ni ₃V ₂O ₈ as a powder photocatalyst for water oxidation ( 31)] but have not been studied or demonstrated as photoanodes. The materials identified by the screening pipeline greatly expand the compendium of known photoanodes with band-gap energy in the 1.2-2.8-eV range and demonstrate the efficacy of integrating high-throughput computational and experimental screening.

We can further evaluate the VO ₄-scaffold hypothesis by quantifying the chemical character of the valence and conduction bands (with respect to the O 2p - V 3d scaffold) for the 91 ternary vanadates from tier 2 that possess a V 3d ⁰ electronic configuration. The results, shown in Fig. 3, confirm our hypothesis, demonstrating that E g can be tuned over a broad 3-eV range through tailoring the VO ₄ band-edge character via orbitals from an additional cation element and crystal structure. Band-gap tuning into the desirable range can be attained via moderate hybridization at either or both of the conduction and valence bands edges, and photoelectrochemical activity is obtained over a diverse set of band-character parameters and band-gap energies. We note that hybridization that enables the tuning of E VBM is particularly critical for optimizing photoanode efficiency.

The results of this discovery pipeline, particularly when combined with existing literature, identify ternary vanadates as a remarkable class of photoanode materials. Out of the 28 known photoanodes from present and previous work, 22 contain V with 16 of those exhibiting the pure VO ₄ motif. Fig. 3 identifies the band-character parameters as important descriptors for understanding and designing improved materials in this class. Additional characterization of the Cu-based phases discovered by this pipeline has been reported by us ( 32) and others ( 33), and we note that the collection of discoveries motivates additional studies, including ( i) the exploitation of band-edge tuning to optimize energetics, ( ii) the characterization of photoactivity over the full solar spectrum, and ( iii) the engineering of electrochemical stability through choice of operating pH and protective or self-passivating layers, all of which are enabled by the range of vanadate chemistries identified by the pipeline. The characterization and optimization of stability is paramount to establishing a deployable photoanode, an area requiring concerted future theory and experimental effort, particularly due to the recently demonstrated propensity for ternary oxides to self-passivate under operational conditions ( 34).

For solar hydrogen generation with a tandem light absorber, device models ( 35) indicate that lowering the photoanode band gap from the BiVO ₄ gap of 2.4 eV to the 1.8-eV value observed with several tier-7 compounds can elicit a 2- to 3-fold enhancement in device efficiency, highlighting the opportunities provided by our discovery of low-band-gap photoanodes. As with well-established photoanodes such as BiVO ₄ and Fe ₂O ₃, each photoanode reported here will require extensive research and development to optimize its solar energy conversion efficiency. This successful implementation of high-throughput materials discovery, where complementary computational and experimental techniques are carefully integrated, introduces a paradigm of materials research in which hypothesis-driven studies can be conducted with significantly improved efficiency.

Acknowledgments

The authors thank Anubhav Jain and Joel Haber for helpful discussions. Computational work was supported by the Materials Project Predictive Modeling Center through the US Department of Energy (DOE), Office of Basic Energy Sciences, Materials Sciences and Engineering Division, under Contract DE-AC02-05CH11231. Experimental work was performed by the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the US DOE (Award DE-SC0004993). Work at the Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences, of the US DOE under Contract DE-AC02-05CH11231. Computational resources were also provided by the DOE through the National Energy Supercomputing Center, a DOE Office of Science User Facility supported by the Office of Science of the US DOE under Contract DE-AC02-05CH11231.

Footnotes

Author contributions: Q.Y., K.A.P., J.M.G., and J.B.N. designed research; Q.Y., J.Y., S.K.S., L.Z., A.S., P.F.N., W.C., and G.L. performed calculations and experiments; Q.Y., S.K.S., K.A.P., J.M.G., and J.B.N. analyzed data; and Q.Y., J.M.G., and J.B.N. wrote the paper with contributions from all authors.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1619940114/-/DCSupplemental.

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Solar fuels photoanode materials discovery by integrating high-throughput theory and experiment

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Combining high-throughput computation and experiment accelerates the discovery of photoelectrocatalysts for water oxidation and explains the origin of their functionality, establishing ternary metal vanadates as a prolific class of photoanode materials for generation of chemical fuels from sunlight. The limited number of known low-band-gap photoelectrocatalytic materials poses a significant challenge for the generation of chemical fuels from sunlight.